A simplified theory for preliminary design of plasma focus devices that starts at the breakdown phase is proposed. For best focus, it is shown that the model can be used to predict the values of the inner and outer electrode radius, inner electrode length, gas pressure, and charging voltage that give minimum possible pinch radius. The theoretical results agree with experimental observations. The model shows that of these parameters, inner electrode length, inner and outer electrode radii play a significant role. An optimum pinch radius ratio of 0.05 to 0.40 was realized. Test results show that the focus strength is dependent on the breakdown conditions via the initial and final velocities of the current sheath during the radial collapse phase and the axial rundown phase. The final axial velocity determines the initial inward radial velocity. Results for a test design are presented.